In a UMTS (Universal Mobile Telecommunications System) network, long-term evolution (LTE) has been standardized for the purpose of further increasing high-speed data rates and providing low delay, etc. (non-patent literature 1). For the purpose of achieving further broadbandization and higher speed from LTE (also called LTE Rel. 8), LTE advanced (which are called LTE Rel. 10, 11, or 12) has been formally specified, and successor systems (also called LTE Rel. 13) thereto have also been studied.
In LTE Rel. 10/11, in order to achieve broadbandization, carrier aggregation (CA) which combines a plurality of component carriers (CCs) is implemented. Each CC is configured as a single unit of the LTE Rel. 8 system frequency band. Furthermore, in CA, a plurality of CCs of the same radio base station (eNB: eNodeB) are configured in the user terminal (UE: User Equipment).
Whereas, in LTE Rel. 12, dual connectivity (DC) is also implemented, in which a plurality of cell groups (CGs) of different radio base stations are configured in a user terminal. Each cell group is configured of at least one cell (CC). In DC, since a plurality of CCs of different radio base stations are combined, DC is also called “Inter-eNB CA”, etc.
Furthermore, LTE Rel. 8 through 12 implement frequency division duplex (FDD) which carries out downlink (DL) transmission and uplink (UL) transmission at different frequencies, and time division duplex (TDD) which periodically switches between downlink transmission and uplink transmission.